Gas valves are commonly used in conjunction with gas-fired appliances for regulating gas flow and/or gas pressure at specific limits established by the manufacturer or by industry standard. Such devices can be used, for example, to establish an upper gas flow limit to prevent over-combustion or fuel-rich combustion within the appliance, or to establish a lower limit to prevent combustion when the supply of gas is insufficient to permit proper operation of the appliance. Examples of gas-fired appliances that may employ such gas valves can include, but are not limited to, water heaters, furnaces, fireplace inserts, gas stoves, gas clothes dryers, gas grills, or any other such device where gas control is desired. Typically, such appliances utilize fuels such as natural gas or liquid propane as the primary fuel source, although other liquid and/or gas fuel sources may be provided depending on the type of appliance to be controlled.
In a fuel-fired, storage-type water heater, for example, a combustion chamber and air plenum are typically disposed adjacent a water storage tank along with a gas valve for controlling the flow of gas into the water heater. A burner element, fuel manifold tube, ignition source, thermocouple, and pilot tube can also be provided as part of the burner system for igniting the fuel within the combustion chamber. During operation, when the temperature of the water within the tank falls below a minimum temperature, metered fuel is introduced via the gas valve through the fuel manifold tube and burner element and into the combustion chamber. The fuel is then ignited by a pilot flame or other ignition source, causing fuel combustion at the burner element. Air may be drawn into the air plenum under the assistance of an air blower, causing the air to mix with the fuel to support the combustion within the combustion chamber. The products of the combusted air-fuel mixture are then fed through a flue or heat exchanger tube in the water tank to heat the water by convection and conduction.
In some instances, it may be desirable for the gas valve to modulate the outlet gas flow in response to a change in the heat-demand or temperature response from the gas-fired appliance. With respect to some gas-fired furnaces, for example, it may be desirable to modulate the gas flow fed to the furnace between a number of different positions to increase or decrease fuel combustion within the burner box based on factors such as air-flow, heat-demand, internal temperature, etc. In some applications, for example, it may be desirable for the gas valve to have fixed low pressure and high pressure limits independent of the appliance size or type of installation, and irrespective of pressure variations at the gas valve inlet. The ability to better control the gas output from the gas valve may be beneficial to increase the efficiency of the appliance to reduce or eliminate energy waste and to prolong the life of the appliance.